1. Field of the Invention
The present invention is directed to a high-frequency power amplifier for feeding an antenna of a nuclear resonance tomography apparatus of the type having at least one amplifier stage that can emit a maximum output power within given frequency band.
2. Description of the Prior Art
The Larmor frequency f.sub.0, i.e. the frequency of a nuclear magnetic resonance signal, is proportional to the magnetic field strength at the location of the nuclei. The proportionality factor is the gyromagnetic constant y. This is utilized in nuclear magnetic resonance tomography systems. The Larmor frequency f.sub.0 therein is basically defined by the basic magnetic field, which, for example, amounts to 0.5 Tesla, 1 Tesla or even 1.5 Tesla. The corresponding frequencies of the nuclear magnetic resonance signal thereby lie on the order of magnitude of 20, 40 or, respectively, 63 MHZ.
For exciting the nuclear spins in a patient disposed in the basic magnetic field is also subject to a radio-frequency field generated by an antenna, whereby the frequency is likewise defined by the gyromagnetic constant .gamma. and the magnetic field strength at the corresponding location in the patient.
A high-frequency power amplifier feeding the antenna must adhere to the boundary conditions explained below. A high-frequency magnetic field strength B.sub.1 that is predetermined independently of the basic magnetic field strength by the relationship B.sub.1 =.alpha./(360.multidot.t.sub.p.multidot..gamma.) is required for the excitation of the nuclear spins with a flip angle .alpha.. The pulse duration t.sub.p can be set by the measuring sequence. Although the high-frequency magnetic field strength is independent of the frequency, the peak transmission power to be supplied by the high-frequency power amplifier increases with the frequency determined by the basic magnetic field. The reason for this is that eddy current losses in the patient increase approximately quadratically with the frequency. In addition, the losses in the antenna increase somewhat with the square root of the frequency. The high-frequency transmission amplifier must have a power output which compensates for (overcomes) these losses in order to generate the desired high-frequency magnetic field strength in the patient, or to effect the desired excitation of the nuclear spins. Different peak transmission powers thus exist for the high-frequency transmission amplifier at different basic magnetic field strengths. Typical values of the peak power are, for example, 10 kW given a frequency of 40.5 MHZ (corresponding to a basic magnetic field strength Bo of 0.95 T) and 15 kW given a frequency of 63.6 MHZ (corresponding to a basic magnetic field strength B.sub.0 of 1.5 T).
Another important dimensioning quantity for the high-frequency power amplifier is the average power that can be applied to the patient, averaged over the pulse repetition time t.sub.R. Independent of the frequency f.sub.0, it is limited by the maximally allowed tissue heating in the patient. A value of 600 W is typical for the average power given an allowable, specific power density of 4 W/kg, a patient weight up to 100 kg and 33% antenna losses. This means that an allowable pulse-duty factor of the high-frequency pulses is lower at higher frequencies than at lower frequencies. For example, the pulse-duty factor amounts to 6% at 40.5 MHZ and 4% at 63.6 MHZ.
A high-frequency power amplifier employable at a number of magnetic resonance frequencies, i.e. in nuclear magnetic resonance tomography apparatus with differing basic magnetic field strength, should thus be able to supply a peak power that increases with the frequency. The average power thereby remains constant. Class B linear amplifiers are usually employed in high-frequency power amplifiers. Their efficiency is optimum when they are operated at full modulation. Only a part of the feed DC voltage is utilized given lower modulation. The direct current and power use of the amplifier thereby drops linearly, but the high-frequency power that is emitted drops with the square of the modulation. The efficiency is then correspondingly reduced by the relationship of peak power to maximum peak power.
When a narrowband amplifier (i.e. the bandwidth is very small compared to the center frequency) has to cover only a single frequency band, then the high-frequency load resistor is selected (or if adjustable, is set) to a value at the output of the last amplifier stage so that full modulation is reached at the maximum peak power. A fixed high-frequency matching network between the amplifier stage that is formed by power transistors or power tubes and the actual amplifier output generally serves this purpose. Differently dimensioned matching networks are required when the narrowband amplifier is to be utilized for a number of frequency bands.
On the other hand, an approximately constant and real load resistor over a frequency range up to several octaves can be achieved given transistor amplifiers with known matching circuits. Such broadband amplifiers do not require differently dimensioned matching networks for the frequency bands lying within these octaves. Given employment in a nuclear magnetic resonance tomography apparatus, the transformation of the high-frequency load must be designed according to the maximum peak load demanded at the highest frequency. The non-optimum efficiency at the lower frequency band must be compensated via corresponding power reserves of the amplifier stage. When, for example, the broadband amplifier is designed for 40 MHZ and 63 MHZ, then the d.c. input power from the supply of such a broadband amplifier is greater by a factor 1.22 for an output power of 10 kW required at 40 MHZ than given optimum design only for 40 MHZ. The theoretical dissipated power is then even greater by the factor 2, which requires double the cooling outlay.
In order to optimize the required input power and the dissipated power to be eliminated, it is desirable that the maximum peak power be exactly equal to the peak power required at the respective operating frequency band. The average values of the input power and of the dissipated power are then frequency-independently defined only by the average output power. The energy consumption required for the operation and the outlay for the power supply and cooling when then be lowest.